Alcohol, hydrogen, ozone and ammonia on A0 to A3, on an Arduino Uno

// Alcohol, hydrogen, ozone and ammonia: an MQ-3, MQ-8, MQ-131 and MQ-137 on
// A0 to A3.
//
//   MQ-3 AO -> A0   MQ-8 AO -> A1   MQ-131 AO -> A2   MQ-137 AO -> A3
//   every VCC -> 5V   every GND -> GND
//
// The MQ-3, MQ-8 and MQ-137 read higher for more gas. The MQ-131 reads LOWER
// for more ozone: ozone is an oxidizing gas and raises the element's
// resistance, where the others lower it. Each reads off at first, while its
// heater warms up: wait for the numbers to settle.
//
// Drag any sensor's gas level up.

void setup() {
  Serial.begin(9600);
  Serial.println("Mokxi Uno: MQ-3, MQ-8, MQ-131 and MQ-137 on A0 to A3");
}

void loop() {
  Serial.print("alcohol ");
  Serial.print(analogRead(A0));
  Serial.print("  H2 ");
  Serial.print(analogRead(A1));
  Serial.print("  O3 ");
  Serial.print(analogRead(A2));
  Serial.print("  NH3 ");
  Serial.println(analogRead(A3));
  delay(1000);
}